A conventional polymer electrolyte fuel cell system will be described below step-by-step.
First, FIG. 16 is a block diagram showing the conventional polymer electrolyte fuel cell system. A cooling water circulating system is provided, in which cooling pure water is supplied from a cooling water tank 2 to a polymer electrolyte fuel cell 1 through a cooling water channel 3 by means of a cooling water pump 4, and the cooling pure water having passed through the fuel cell 1 is cooled in a heat exchanger 5, such as a radiator, and then returns to the cooling water tank 2.
There is a problem in that conductive ions are eluted into the cooling pure water from the heat exchanger 5, which cools the cooling water, and the increased conductive ions cause short-circuit in the fuel cell 1, thereby reducing the amount of electric power generation. Thus, the conductive ions need to be removed from the cooling water, and to that end, an ion removal filter 6, which includes an ion exchange resin for removing the conductive ions eluted from the heat exchanger 5, is provided in the cooling water channel 3, the ion removal filter.
With the ion removal filter 6 provided in the cooling water channel 3, when the fuel cell 1 requires a large amount of water during high load operation, pressure loss of the cooling water is increased, therefore a bypass channel 7 is provided to reduce the pressure loss; and when a small amount of water during low load operation, that is not affected by the pressure loss in the ion removal filter 6, a flow control valve 8 is provided in the bypass channel 7, and the flow control valve enables the cooling pure water to be actively passed through the ion removal filter 6.
Although not shown, the water in the cooling water tank is reduced in the course of circulation, and thus, water needs to be supplied to the tank from outside. The supplied water may be water introduced from outside, such as tap water, or recycled water in the system.
As for the tap water, an element harmful to the human body is removed to a quite low concentration for the water to be suitable for drinking. However, for a hard water constituent, such as calcium and magnesium, and salinity, such as sodium and chlorine, which are not harmful to the human body, a significantly high concentration is permitted. Furthermore, for sterilization, sodium hypochlorite is added to the water in such a manner that the concentration thereof is equal to or higher than a certain value at the tap. Thus, the water is quite inferior in purity. Besides, in the case of water introduced from another source, such as ground water, a large quantity of organic acid or hard water constituent is contained therein. Thus, it is the same as the tap water in that it requires adequate purification.
On the other hand, the water recycled in the system is generated by reaction of hydrogen (H2) and oxygen (O2) in the fuel cell during electric power generation, so that a high purity can be attained in principle. However, in order to construct a practical system, some parts, such as a pipe and a valve, have to be made of metal in terms of safety or thermal efficiency, and it is difficult to entirely prevent metal ions from being eluted into the recycled water. Furthermore, the recycled water may be brought into contact with air, and carbon dioxide may be eluted therein to produce carbonate ions, thereby increasing the conductivity. Therefore, also in the case where the recycled water is used as the cooling water for a hydrogen generator or fuel cell, purification using the ion exchange resin or the like is needed.
Generally, the polymer electrolyte fuel cell system is operated with the cooling water at 70 to 80° C. circulated therein. Since the cooling water channel 3 of the polymer electrolyte fuel cell system shown in FIG. 16 is a closed type, when the system is operated, the temperature of the cooling water is changed from an ordinary temperature before operation to a high temperature during electric power generation, and accordingly, a pressure in the cooling water tank 2 and a pressure of the water in the cooling water channel 3 are increased. When the system operation is stopped, the temperature of the cooling water is changed from the high temperature during electric power generation to the ordinary temperature after operation, and accordingly, the pressure in the cooling water tank 2 and the pressure of the water in the cooling water channel 3 are decreased. Thus, there is a first problem in that the cooling water tank 2 and the cooling water channel 3 need to be constructed so as to withstand the pressure variation caused by the temperature variation.
Besides, since the ion removal filter 6 is provided in the cooling water channel 3, the increase of the flow rate of the cooling water results in the increase of the amount of the cooling water passing through the ion removal filter 6, whereby the pressure loss in the cooling water channel 3 is increased. In the conventional example, to avoid such a situation, the bypass channel 7 and the flow control valve 8 is used. However, there is a second problem in that the cost is increased by the increase in the number of components. To address this problem, the capability of the cooling water pump 4 may be enhanced. In this case, however, the cost as well as power consumption by auxiliary machinery for operating the system are increased, whereby the efficiency of the whole system is reduced. Thus, it cannot be said a sufficient solution.
In addition, there is a third problem as follows. Although most of the ion exchange resins (anion exchange resins, in particular) have a relatively low durable temperature, the temperature of the cooling water that cools the polymer electrolyte fuel cell 1 is about 70 to 80° C. Therefore, the ion exchange resin in the ion removal filter 6 provided in the cooling water channel 3 is thermally degraded due to long time operation under a strict condition in terms of the durable temperature, whereby the life thereof tends to be shortened. Furthermore, even if the quality of the cooling water is good, the cooling water at 70 to 80° C. flows. This is inefficient and further shortens the life of the ion exchange resin in the ion removal filter 6.
As described above, when the fuel cell system is to be put into practical use, water introduced from outside, which has an insufficient purity, has to be used. Therefore, the ion exchange resin is degraded rapidly, and a small device would have a shortened life and need frequent maintenance, and thus, the running cost thereof would be increased. In order to extend the life of the ion exchange resin, a method of pre-purifying the water by means of a reverse osmosis membrane provided in the preceding stage and then purifying the same water by means of the ion exchange resin is often used in ultra pure water production or the like. As disclosed in Japanese Patent Laid-Open No. 10-235396, there is also a purified water production apparatus for a fuel cell in which water is passed through the ion exchange resin after being purified to a certain degree through the reverse osmosis membrane. Here, the entire disclosure of the above-described Japanese Patent Laid-Open No. 10-235396 is incorporated herein by reference in its entirety.
Here, there is a fourth problem as follows. The reverse osmosis membrane is a membrane with micropores. While most of impurities cannot pass through the membrane, water molecule can pass through it. Therefore, water with a high purity exudes to the other side of the membrane. The impurities not passing through the membrane are continuously discharged in the form of condensed water. The higher the pressure of the water supplied, or the higher the temperature thereof, the larger amount of water can be purified. However, the amount of the condensed water discharged is also increased, and the running cost for the discharged water is increased accordingly. To eliminate the discharged water, another structure, for example, a channel to return the discharged water to the raw water for re-purification needs to be provided.